On Sun, 20 Jul 2003 22:58:47 -0700, "Roger Shoaf"
wrote:

As I understand the physics, air is a fluid and the helium balloon being
lighter than the air around it floats just as a drop of oil will float if
released from the bottom of a volume of water.

But, mechanically, "how" does it float. You almost answered it below.

Not as easy as one might think.

And a second one is why when you have a helium balloon in your car and
put the brakes on to stop, the balloon will float to the back of the
car instead of flying to the front like everything else. Again, what
are the mechanics that make it do so?

Again the air being heaver than the balloon the air has more mass. When
this mass is in motion, pressure is equal on all sides of the balloon. When
the brakes are applied the air inside the car keeps moving at the speed the
car was going until it hits the windshield. As air bunches up in the front
part of the car, there is more air pressure at the front of the car and less
at the back so the pressure in the front pushes the lighter balloon
backwards.

It took a little time for a friend and I to realize how a balloon
floats. I think about half a Big Mac at McDonalds one day. You're
correct here. It really puzzled me for awhile when I first noticed
this phenonemen (sp). I have a lot of fun with kids when I tell them
this, they usually go beserk and get real excited when they see it
happen.

The reason why a balloon floats in still air is because of the
pressure gradient between the top and the bottom of the balloon. Most
people don't think there is enough of a gradient to make a balloon
float but there is. They usually think of pressure drops between sea
level and the top of a mountain, not one over just a foot of height.
If the pressure was the same it would sink.

John

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